Previous work (1,2) in our laboratory showed that the addition of protein hydrolysates or certain amino acids to a 9 per cent casein-sucrose ration deficient in niacin resulted in a marked reduction in the growth rate of rats. The further addition of either niacin or tryptophan restored growth to a rate equal to or exceeding that of rats receiving the unsupplemented basal ration. Subsequent experiments (3) have shown that a mixture of amino acids simulating 2 per cent acid-hydrolyzed casein is

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... qually effective in inducing the niacin deficiency. The experiments reported in this paper deal with the effect of individual amino acids and the possible mechanism of their action. EXPERIMENTAL The procedures used in these experiments were similar to those employed in previous studies. 3 week-old male Sprague-Dawley rats, kept in individual cages, were fed ad l&turn over a 5 week experimental period, and weighed at weekly intervals. The basal diet had the following composition: sucrose 81.8 per cent, corn oil 5 per cent, casein' 9 per cent, Salts IV 4 per cent (4), L-cystine 0.2 per cent, and vitamins added as a dry mixture dilut,ed with sucrose to provide the following quantities per 100 gm. of diet: thiamine 0.2 mg., riboflavin 0.3 mg., pyridoxine 0.25 mg., calcium m-pantothenate 2 mg., choline chloride 100 mg., inositol 10 mg., biotin 0.01 mg., and folic acid 0.02 mg. Halibut liver oil fortified with vitamins E and K and diluted with corn oil was used as a source of the fat-soluble vitamins.